The present invention relates to a Zeeman atomic absorption spectrophotometer.
Atomic absorption spectrophotometers have been widely used to quantitatively analyze metal elements contained in waste water, foods and urine. Among the atomic absorption spectrophotometers, an atomic absorption spectrophotometer which utilizes the Zeeman effect has high sensitivity and is capable of analyzing trace amounts of metals.
According to the Zeeman atomic absorptiometric method, a magnetic field of about 10 KG is applied to an atomized sample vapor, and the resonance absorption by atoms is measured by using two kinds of linearly polarized light with planes of polarization which intersect at right angles to each other. The polarized light (P //) having a plane of oscillation parallel with the magnetic field serves as a sample light beam, and the polarized light (P.perp.) having a plane of oscillation perpendicular to the magnetic field serves as a reference light beam. This method is capable of precisely correcting the background absorption and features a very stable base line owing to its complete double beam method, and hence has drawn attention among many researchers in recent years as an extremely reliable and effective analytical method. The Zeeman effect can be utilized in several different ways. For example, in addition to the above-mentioned method by which the magnetic field of a predetermined intensity is applied to the sample vapor, the magnetic field may be applied to the light source, or the intensity of the magnetic field may be modulated. However, a problem which commonly develops in the Zeeman atomic absorptiometric methods is that the calibration curve drops in very high concentration regions.
The calibration curve drops and inverts in regions where the concentration is greater than a concentration Cp which gives a maximum absorbance Ap. There is no problem with regard to the reliability of the measured values as long as the concentrations measured by the Zeeman atomic absorptiometric method are smaller than the concentration Cp. Since the calibration curve is given by a divalent function, however, an absorbance A.sub.1 obtained by measuring an unknown sample does not guarantee that the sample concentration is C.sub.1. There is also a possibility that the specimen concentration may be C.sub.2 that is greater than C.sub.1. As far as the calibration curve is given by a divalent function, it is quite impossible to determine whether it is C.sub.1 or C.sub.2 by the existing method. Furthermore, in regions of relatively high concentrations the calibration curve is so curved that it is difficult to analyze the concentrations with sufficient precision.
Theoretically, the drawback phenomenon can be explained as described below. Namely, according to the Zeeman atomic absorptiometric method of the type in which the magnetic field is applied to the sample atomizing portion, the background is corrected by using light from the light source which has the same plane of polarization as the component that does not undergo Zeeman split and light from the light source which has a right angle to the component, and a differential signal between the two components is obtained. In the high concentration regions, the absorption by the atoms saturates, whereas the background absorption increases linearly. Consequently, the differential signal is saturated, and inherently in the atomic absorptiometric method, a curving factor is added to the calibration curve giving rise to the occurrence of the drawback phenomenon.
It has been reported that a variety of types of Zeeman atomic absorptiometric methods can theoretically be realized, but in practice only one kind of Zeeman atomic absorption spectrophotometer has been produced in the world. This Zeeman atomic absorption spectrophotometer employs a graphite atomizer in a portion where the sample is to be atomized, consisting of a graphite tube and a power supply for supplying current to the tube. The sample is introduced into the tube and is atomized by the Joule heat produced by the electric current. U.S. patent application Ser. No. 236,995, filed Feb. 23, 1981, now U.S. Pat. No. 4,377,342, discloses a method which eliminates the above-mentioned drawback phenomenon inherent in the Zeeman atomic absorption spectrophotometer of this type.
The inventors of the present invention have also encountered the same problem during the course of developing a new Zeeman atomic absorption spectrophotometer using a chemical flame in the sample-atomizing portion.
In the generally employed atomic absorption spectrophotometer employing a chemical flame in the sample-atomizing portion, the absorbance signal during a predetermined period of time is averaged in order to remove noise which stems from the flame. The concentration is then found from the average value which takes the form of a divalent function.